It seems Google has pulled the new line completely from their guidelines. It is unclear why but I suspect after I emailed them they reviewed it and found it to be confusing as well.

The new guideline that was added for about 24 hours read:

Make efforts to ensure that a robots.txt file does not block a destination URL for a Google Ad product. Adding such a block can disable or disadvantage the Ad.

As I explained, it was confusing because Google asks you specifically to block other ads from being crawled. But here, Google wants you to allow Google to crawl those ads. You and I understand why, because Google uses landing page quality score as part of AdWords ranking but still, it is confusing how they worded it.

Now, the language and the bullet point, is completely gone. The guidelines are back to how they were the day before.

We were considering something along these lines since advertising programs (not just ours) often need to crawl the landing pages, but we decided not to add it to the webmaster guidelines. Sorry for the confusion there.